linux-kernel - Re: [PATCH v2 0/8] Add rcu_sync infrastructure to avoid

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Message-ID: <20150826125215.GA25142@redhat.com>
Date:	Wed, 26 Aug 2015 14:52:15 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/8] Add rcu_sync infrastructure to avoid
	_expedited() in percpu-rwsem

On 08/26, Oleg Nesterov wrote:
>
> On 08/25, Paul E. McKenney wrote:
> >
> > On Mon, Aug 24, 2015 at 05:34:31PM +0200, Oleg Nesterov wrote:
> > >
> > > I booted the kernel with the additional patch below, and nothing bad has
> > > happened, it continues to print
> > >
> > > 	Writes:  Total: 2  Max/Min: 0/0   Fail: 0
> > > 	Reads :  Total: 2  Max/Min: 0/0   Fail: 0
> > >
> > > However, I do not know what this code actually does, so currently I have
> > > no idea if this test makes any sense for percpu_rw_semaphore.
> >
> > Actually, unless I am really confused, that does not look good...
> >
> > I would expect something like this, from a run with rwsem_lock:
> >
> > 	[   16.336057] Writes:  Total: 473  Max/Min: 0/0   Fail: 0
> > 	[   16.337615] Reads :  Total: 219  Max/Min: 0/0   Fail: 0
> > 	[   31.338152] Writes:  Total: 959  Max/Min: 0/0   Fail: 0
> > 	[   31.339114] Reads :  Total: 437  Max/Min: 0/0   Fail: 0
> > 	[   46.340167] Writes:  Total: 1365  Max/Min: 0/0   Fail: 0
> > 	[   46.341952] Reads :  Total: 653  Max/Min: 0/0   Fail: 0
> > 	[   61.343027] Writes:  Total: 1795  Max/Min: 0/0   Fail: 0
> > 	[   61.343968] Reads :  Total: 865  Max/Min: 0/0   Fail: 0
> > 	[   76.344034] Writes:  Total: 2220  Max/Min: 0/0   Fail: 0
> > 	[   76.345243] Reads :  Total: 1071  Max/Min: 0/0   Fail: 0
> >
> > The "Total" should increase for writes and for reads -- if you are
> > just seeing "Total: 2" over and over, that indicates that either
> > the torture test or rcu_sync got stuck somewhere.
>
> Hmm. I reverted the change in locktorture.c , and I see the same
> numbers when I boot the kernel with
>
> 	locktorture.verbose=1 locktorture.torture_type=rwsem_lock
>
> parameters.
>
> 	Writes:  Total: 2  Max/Min: 0/0   Fail: 0
> 	Reads :  Total: 2  Max/Min: 0/0   Fail: 0
>
> "Total" doesn't grow. Looks like something is wrong with locktorture.
> I'll try to re-check...

Heh ;) torture threads spin in stutter_wait(). Added another parameter,

	locktorture.torture_runnable=1

now I see the similar numbers

	Writes:  Total: 1242  Max/Min: 0/0   Fail: 0
	Reads :  Total: 892  Max/Min: 0/0   Fail: 0
	Writes:  Total: 2485  Max/Min: 0/0   Fail: 0
	Reads :  Total: 1796  Max/Min: 0/0   Fail: 0
	Writes:  Total: 3786  Max/Min: 0/0   Fail: 0
	Reads :  Total: 2713  Max/Min: 0/0   Fail: 0
	Writes:  Total: 5045  Max/Min: 0/0   Fail: 0
	Reads :  Total: 3636  Max/Min: 0/0   Fail: 0

with or without s/rw_semaphore/percpu_rw_semaphore/ change in locktorture.c

Oleg.

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